No, 5htp won't do anything for your GABA levels. 5htp will increase your serotonin levels, if you have enough b6 and niacin in your system.

You might try supplementing some glutamine. Most amino acids should not be taken long term without consulting with a medical professional because taking one or two amino acids may cause deficiencies of other amino acids over time.

But glutamine is a non-essential amino acid which is used as the only energy source for your GI track. I don't think that taking glutamine should be a problem because this amino acid is often used for GI problems especially leaky gut and ulcers, but other problems related to GI healing. Of course you should be taking a multi-vitamin besides the glutamine at the very least.

I take one gram of glutamine in between meals (in capsule form). You can try the capsules (500 mg.) or the powder which has a slightly sweet taste in water. But the powder may be hard to take a relatively low dose like one or grams. The powder is great for taking several grams at a time. And the powder is best for your GI track because when you mix it with water you can feel it soothe your GI track as it go down. The powder is good to have on hand if you have any type of GI problems that require healing the lining.

Glutamine in high doses can cause a manic episode in some people. But that might require more than 6 grams per day. Taking a high dose at night (several grams) before bedtime may also cause alertness and interfere with sleep. It is probably best to take several smaller doses throughout the day instead of one big or two big doses per day.

http://www.vitaminstuff.com/glutamine.html
Inside the brain glutamine may be converted into glutamic acid, another amino acid that helps sustain proper brain function; it also increases levels of gamma-aminobutyric acid (GABA), an amino acid that acts as a neurotransmitter in the body. L-glutamine supplements may improve mental function and have been used to treat epilepsy, depression, schizophrenia, and senility. Glutamine is also an important source of energy for the nervous system. If the brain is not receiving enough glucose, it compensates by increasing glutamine metabolism for energy. Glutamine users often report more energy, less fatigue and better mood.

Glutamine is good for you your brain because the brain uses only two sources for energy, glucose and glutamine.